This invention relates to an antenna device used for air traffic radar.
There are two kinds of air traffic control radars. One kind is a primary surveillance radar (PSR) making use of signals reflected from an airplane for locating it. Examples of this type of radar are an airport surveillance radar (ASR) or an air route surveillance radar (ARSR). Another kind of traffic control radar is a secondary surveillance radar (SSR). An SSR utilizes a response signal which may include airplane identification information transmitted from an airplane's transponder.
Both these radar systems are often used together, and their antennas are used in combination. For example, the ARSR system, the function of which is to suppress clutter and which uses a dual beam type reflector antenna radiating both low and high beams, and the SSR antenna, which radiates a beam of a narrow width in the horizontal plane and uses an array antenna, are installed together, the SSR antenna being mounted on top of the reflector of the ARSR antenna.
However, there has recently been a need to use an antenna having a vertical plane radiation pattern having sharp cut-off characteristic even in the SSR system in order to avoid lobing due to clutter. Therefore, it is sometimes necessary to use a reflector antenna having a large aperture like the SSR antenna. In such a case it is difficult to install the SSR antenna on top of the reflector of the ARSR antenna.
The reflector of the ARSR antenna is constructed to provide for a vertical plane radiation pattern having a sharp cut-off characteristic at approximately 1.3 GHz. Therefore, if the SSR system covers a band of 1.03 to 1.09 GHz, for instance, the reflector of an ARSR antenna can be commonly used for both the ARSR and SSR radar systems. To this end, the primary radiator of the SSR system may be installed in the neighborhood of the primary radiator of the ARSR system. However, the primary radiator of the dual beam system ARSR antenna includes a high beam horn disposed below the low beam horn. There are the problems associated with how and where the SSR primary radiator is located in relation to these high and low beam horns.
Where a single SSR primary radiator is arranged adjacent to the low beam horn, it is defocused in the Azimuth plane, and therefore beam shift or beam skew occurs in the horizontal plane radiation pattern of the SSR antenna. This causes a shift of the beam nose in the horizontal plane radiation patterns of the SSR and ARSR antennas and makes the mono-pulse angle measurement impossible. By using two horns arranged, for example, in an Azimuth plane, a mono-pulse angle measurement is carried out by obtaining sum and difference signals on the output of the respective horns. In this case, it is required that sum and difference patterns be symmetrical with respect to the antenna axis on the azimuth plane. Where two SSR primary radiators are disposed on opposite sides of the low beam horn, the low beam horn being large in size, the SSR primary radiators are spaced too far apart, giving rise to a beam split in the SSR system antenna in the horizontal plane radiation pattern and making the mono-pulse angle measurement impossible. This arrangement is not suitable for the SSR antenna.